Scientists propose a solution to a critical
barrier to producing fusion

April 23, 2012, Plainsboro, NJ —
Physicists have discovered a possible solution to a mystery that
has long baffled researchers working to harness fusion. If
confirmed by experiment, the finding could help scientists
eliminate a major impediment to the development of fusion as a
clean and abundant source of energy for producing electric
power.

An in-depth analysis by scientists from the U.S.
Department of Energy’s Princeton Plasma Physics
Laboratory (PPPL) zeroed in on tiny, bubble-like islands that
appear in the hot, charged gases—or
plasmas—during experiments. These minute islands collect
impurities that cool the plasma. And it is these islands, the
scientists report in the April 20 issue of Physical Review Letters,
that are at the root of a long-standing problem known as the
“density limit” that can prevent fusion
reactors from operating at maximum efficiency.

Fusion occurs when plasmas become hot and dense
enough for the atomic nuclei contained within the hot gas to
combine and release energy. But when the plasmas in experimental
reactors called tokamaks reach the mysterious density limit, they
can spiral apart into a flash of light. “The big mystery
is why adding more heating power to the plasma doesn’t
get you to higher density,” said David A. Gates, a
principal research physicist at PPPL and co-author of the proposed
solution with Luis Delgado-Aparicio, a post-doctoral fellow at PPPL
and a visiting scientist at MIT’s Plasma Science Fusion
Center. “This is critical because density is the key
parameter in reaching fusion and people have been puzzling about
this for 30 or 40 years.”

Shown above from
left to right are physicists Luis Delgado-Aparicio and David Gates,
who have proposed a solution to a major impediment to fusion
reactors. (Photo credit: Elle Starkman, PPPL Office of
Communications)

The scientists hit upon their theory in what
Gates called “a 10-minute ‘Aha!’
moment.” Working out equations on a whiteboard in
Gates’ office, the physicists focused on the islands and
the impurities that drive away energy. The impurities stem from
particles that the plasma kicks up from the tokamak wall.
“When you hit this magical density limit, the islands
grow and coalesce and the plasma ends up in a
disruption,” says Delgado-Aparacio.

These islands actually inflict double damage,
the scientists said. Besides cooling the plasma, the islands act as
shields that block out added power. The balance tips when more
power escapes from the islands than researchers can pump into the
plasma through a process called ohmic heating—the same
process that heats a toaster when electricity passes through it.
When the islands grow large enough, the electric current that helps
to heat and confine the plasma collapses, allowing the plasma to
fly apart.

Gates and Delgado-Aparicio now hope to test
their theory with experiments on a tokamak called Alcator C-Mod at
MIT, and on the DIII-D tokamak at General Atomics in San Diego.
Among other things, they intend to see if injecting power directly
into the islands will lead to higher density. If so, that could
help future tokamaks reach the extreme density and
100-million-degree temperatures that fusion requires.

The scientists’ theory represents a
fresh approach to the density limit, which also is known as the
“Greenwald limit” after MIT physicist Martin
Greenwald, who has derived an equation that describes it. Greenwald
has another potential explanation of the source of the limit. He
thinks it may occur when turbulence creates fluctuations that cool
the edge of the plasma and squeeze too much current into too little
space in the core of the plasma, causing the current to become
unstable and crash. “There is a fair amount of evidence
for this,” he said. However, he added, “We
don’t have a nice story with a beginning and end and we
should always be open to new ideas.”

Gates and Delgado-Aparicio pieced together their
model from a variety of clues that have developed in recent
decades. Gates first heard of the density limit while working as a
post-doctoral fellow at the Culham Centre for Fusion Energy in
Abingdon, England, in 1993. The limit had previously been named for
Culham scientist Jan Hugill, who described it to Gates in
detail.

Separately, papers on plasma islands were
beginning to surface in scientific circles. French physicist
Paul-Henri Rebut described radiation-driven islands in a mid-1980s
conference paper, but not in a periodical. German physicist
Wolfgang Suttrop speculated a decade later that the islands were
associated with the density limit. “The paper he wrote
was actually the trigger for our idea, but he didn’t
relate the islands directly to the Greenwald limit,” said
Gates, who had worked with Suttrop on a tokamak experiment at the
Max Planck Institute for Plasma Physics in Garching, Germany, in
1996 before joining PPPL the following year.

In early 2011, the topic of plasma islands had
mostly receded from Gates’ mind. But a talk by
Delgado-Aparicio about the possibility of such islands erupting in
the plasmas contained within the Alcator C-Mod tokamak reignited
his interest. Delgado-Aparicio spoke of corkscrew-shaped phenomena
called snakes that had first been been observed by PPPL scientists
in the 1980s and initially reported by German physicist Arthur
Weller.

Intrigued by the talk, Gates urged
Delgado-Aparicio to read the papers on islands by Rebut and
Suttrop. An email from Delgado-Aparicio landed in Gates’
in-box some eight months later. In it was a paper that described
the behavior of snakes in a way that fit nicely with the C-Mod
data. “I said, ‘Wow! He’s made a lot
of progress,’” Gates remembers. “I
said, ‘You should come down and talk about
this.’”

What most excited Gates was an equation for the
growth of islands that hinted at the density limit by modifying a
formula that British physicist Paul Harding Rutherford had derived
back in the 1980s. “I thought, ‘If Wolfgang
(Suttrop) was right about the islands, this equation should be
telling us the Greenwald limit,” Gates said.
“So when Luis arrived I pulled him into my
office.”

Then a curious thing happened. “It
turns out that we didn’t even need the entire
equation,” Gates said. “It was much simpler
than that.” By focusing solely on the density of the
electrons in a plasma and the heat radiating from the islands, the
researchers devised a formula for when the heat loss would surpass
the electron density. That in turn pinpointed a possible mechanism
behind the Greenwald limit.

Delgado-Aparicio became so absorbed in the
scientists’ new ideas that he missed several turnoffs
while driving back to Cambridge that night.
“It’s intriguing to try to explain Mother
Nature,” he said. “When you understand a theory
you can try to find a way to beat it. By that I mean find a way to
work at densities higher than the limit.”

Conquering the limit could provide essential
improvements for future tokamaks that will need to produce
self-sustaining fusion reactions, or “burning
plasmas,” to generate electric power. Such machines
include proposed successors to ITER, a $20 billion experimental
reactor that is being built in Cadarache, France, by the European
Union, the United States and five other countries.

Why hadn’t researchers pieced together
a similar theory of the density-limit puzzle before? The answer,
says Gates, lies in how ideas percolate through the scientific
community. “The radiation-driven islands idea never got a
lot of press,” he says. “People thought of them
as curiosities. The way we disseminate information is through
publications, and this idea had a weak initial push.”

By John Greenwald

About PPPL and DOE

PPPL, in Plainsboro, N.J., is devoted both to
creating new knowledge about the physics of plasmas —
ultra-hot, charged gases — and to developing practical
solutions for the creation of fusion energy. Through the process of
fusion, which is constantly occurring in the sun and other stars,
energy is created when the nuclei of two lightweight atoms, such as
those of hydrogen, combine in plasma at very high temperatures.
When this happens, a burst of energy is released, which can be used
to generate electricity.

Princeton Plasma Physics Laboratory (www.pppl.gov)is managed by Princeton
University for the U.S. Department of Energy’s Office of
Science. DOE’s Office of Science is the single largest
supporter of basic research in the physical sciences in the United
States, and is working to address some of the most pressing
challenges of our time. For more information, please visit science.energy.gov.